1. Field of the Invention
The invention relates to a method for identifying a gas leak between anode and cathode gas areas of a PEM fuel cell, and to a fuel cell system for carrying out the method.
It is known that during the electrolysis of water, water molecules are broken down by electric currents into hydrogen and oxygen. In the fuel cell, that process takes place in the opposite direction. During the electrochemical combination of hydrogen and oxygen to form water, electric current is produced with high efficiency, and to be precise, if pure hydrogen is used as the combustion gas, that is done without any emission of hazardous materials or carbon dioxide. Even with technical combustion gases, such as natural gas or coal gas, and using air or air enriched with O.sub.2 instead of pure oxygen, a fuel cell produces a considerably smaller amount of hazardous materials and less CO.sub.2 than other energy producers which operate with fossil energy sources. The technical implementation of the principle of the fuel cell has led to widely differing solutions, to be precise with different types of electrolytes and with operating temperatures between 80.degree. C. and 1000.degree. C. The fuel cells are split into low-temperature, medium-temperature and high-temperature fuel cells depending on their operating temperature and are furthermore respectively distinguished by different technical embodiments.
In addition to those fundamental advantages, a fuel cell having a solid electrolyte composed of plastic (polymer electrolyte membrane or PEM) has other positive characteristics, such as a low operating temperature (less than or equal to 80.degree. C.), a good overload behavior, little voltage degradation, long life, good load and temperature cycling response, and lack of any liquid corrosive electrolyte. Furthermore, it is also suitable for operation with air from the environment, instead of oxygen. All of those characteristics make the PEM fuel cell, which can be operated with air, a virtually ideal electrical source, for example for electrical power for motor vehicles, without any emissions.
A fuel cell block, which is also referred to as a "stack" in the specialist literature, as a rule is composed of a multiplicity of fuel cells stacked on top of each other and having a planar construction. Since the fuel cell block cannot be operated on its own, the fuel cell block, an operating section and an associated electronics module generally are combined to form a fuel cell module. The operating section contains devices for supplying operating material or agents, for example hydrogen (H.sub.2), oxygen (O.sub.2) or air from the environment, for removing product water, for dissipating heat losses, for moistening the operating material or agents and for extracting inert gas constituents.
During operation of a PEM fuel cell module, any defect that occurs in an individual PEM fuel cell leads to a disturbance in the operation of the entire PEM fuel cell module. For example, a gas escape from an anode gas area to a cathode gas area of the PEM fuel cell due to a leak in a membrane electrode unit can lead to a thermal reaction between hydrogen (H.sub.2) and oxygen (O.sub.2) being initiated on the catalyst. Rapid identification of the gas leak between the anode and the cathode gas areas and identification of such a defective fuel cell in the fuel cell module following such an occurrence, have been found to be problematic.
In a method known from the prior art for identifying a gas leak between the anode and cathode sides of the fuel cell, a pressure difference is produced between those sides, and a rate of change of the pressure difference is evaluated. Pressures are thus measured in that method. That method uses an integral measurement over a plurality of fuel cells in the fuel cell module, that is to say it is not possible to identify an individual defective fuel cell in the fuel cell module. Furthermore, the method is unsuitable for identifying small leaks in a fuel cell module, due to its low sensitivity.